Continuous anodizing process and apparatus



Dec. 19, 1967 w. E. COOKE ETAL 3,359,189

CONTINUOUS ANODIZING PROCESS AND APPARATUS 5 Sheets-Sheet 1 Filed Feb. 4, 1964 071/5/7/0/5 Emesf Coa/re W/U/am Eger/on Bury Pau/ 5/77/75 Affomey Dec. 19, 1967 w. E. COOKE ETAL 3,359,189

CONTINUOUS ANODIZING PROCESS AND APPARATUS 5 Sheets-Sheet 2 Filed Feb. 4, 1964 5065/ Coo/re y w W. 5 m @w m 2% 5 g E W 0 0 W M D R w NM vm CONTINUOUS ANODIZING PROCESS AND APPARATUS A f/omey Dec. 19, 1967 3,359,189

CONTINUOUS ANODIZING PROCESS AND APPARATUS W. E. COOKE ETAL 5 Sheets-Sheet 5 Filed Feb. 4, 1964 6 y k QC m w 5 w 0 m 66 n m @Q QQ MQ MWMS A Q Q N 6 n S \V/ V ff U Q31 m3 5 7 n E d m NQ mW .m

50mm KEMP V PZAIOOQ United States Patent 3,359,189 CONTHNUOUS ANODIZING PROCESS AND APPARATUS William Ernest Cooke, Kingston, Ontario, David Egerton Bury, Toronto, Ontario, and Paul Smits, Kingston, 0ntario, Canada, assignors to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a corporation of Canada Filed Feb. 4, 1964, Ser. No. 342,450 Claims. (Cl. 20428) ABSTRACT OF THE DISCLOSURE A method for continuous production of porous anodic coatings on elongated articles such as aluminum strip embraces anodizing the passing strip in an acidic solution, e.g. sulfuric acid, which produces such coating, while advancing the electrolyte along the surfaces of the strip with effective turbulence uniformly created throughout the exposed area so that heat is effectively and uniformly removed and dissolution of the coating oxide is prevented. Preferably the strip first traverses a cleaning region of similar electrolyte where it is likewise submerged and subjected to like turbulent flow, the anodizing current being there passed to the strip, and then from the strip in the anodizing region, travel of the strip between areas being maintained in submerged manner through an intermediate orifice. Differential anodizing of opposite surfaces of a strip is obtainable by differently controlling the supply of electrical energy, eg the current that passes from the strip to electrodes on opposite sides of it.

Apparatus for continuous anodizing includes an integrated assembly of elongated horizontal cleaning and anodizing tanks each with liquid entrance and exit conduits at opposite ends to effectuate high speed circulation of liquid in turbulent manner along the surfaces of the passing strip. Side spacer means and upper and lower spacer grids provide for guiding the strip in centered relalation between the upper and lower walls of each section, which may constitute electrodes. The strip enters and leaves through compressible end seals, while means preferably individual to the tanks are provided for circulating the electrolyte and controlling its temperature, the system including insulated piping in the connections to the tank inlet and outlet parts to prevent significant electric current loss. A switching arrangement provides a selectable variety of connections for choice of cleaning and anodizing actions.

This invention relates to the production of anodic coatings or films on aluminum, the term aluminum being employed to include aluminum base alloys which, like pure aluminum, are susceptible of being anodized to produce porous type oxide films. More particularly, the invention is directed to apparatus and procedure for continuous anodizing of extended aluminum articles, meaning sheet, strip, wire, rod or the like, being articles that are greatly elongated and may therefore be caused to traverse continuously one or more baths wherein anodizing or other treatment is performed. In a further sense, the present improvements are concerned with apparatus for continuous electrolytic treatment of elongated aluminum members, including both anodic and other operations, a particular aim being to afford improved apparatus for performing successive treatments on the passing member, up to very rapid rates of travel of the latter.

The invention is thus especially designed to achieve relatively high speed operation, whereby anodic films of desirable character are applied to elongated aluminum articles, without difficulties of imperfections in the film,

overheating of the metal, poor etliciency and uncertainty in control, such as have attended at least a number of prior efforts to achieve rapid anodic treatment, or more particularly, a continuous anodizing operation which is economically satisfactory. A special object is the improvement of procedure for applying relatively thick, porous type anodic coatings, i.e. of a thickness of 0.1 mil and notably upwards thereof, so as to achieve such coatings in a rapid, continuous manner that is believed to have been unattainable in prior practice.

A further object of the invention is to provide apparatus adapted for achievement of results as outlined above, which by adjustment or modification of its structure may be readily adapted for a variety of types of treatment, e.g. as to sheet in strip form, or as to wire or the like, or with respect to the nature of the coating, a special feature being adaptability of the equipment and process to the production of anodic coating on both sides or only one side, of strip material (e.g. sheet). Another object is to provide, in continuous anodizing systems or methods, for simultaneous treatment of opposite sides of a strip in a different way, i.e. to create anodic coatings having different characteristics of thickness, porosity or the like, such operation being conveniently herein described as differential anodizing.

For the above objects of rapid, continuous production of relatively thick anodic coatings, the process of the invention comprises advancing the aluminum article lengthwise through, and in eifect, in submergence in aqueous electrolyte, e.g. as contained in or confined by an appropriate elongated vessel, while maintaining such electrolyte, over all exposed surfaces of the article, in continuous turbulent flow lengthwise of the path of travel of such article and while passing electric current from the article into and through the electrolyte at relatively high density, i.e. at least about amperes per square foot of exposed surface, and more specifically at densities of about 300 amperes per square foot and upwards. Thus in the case of continuous aluminum sheet or equivalent article, the aqueous liquid is arranged in two bodies respectively in contact with opposite sides of the sheet, and in continuous flow along such sides, being brought from and returned to appropriate pumping and temperature-controlling instrumentalities whereby there is continuous recirculation. In all instances the entire exposed area of the aluminum article (e.g. both faces of the moving sheet) is kept in contact with the turbulently flowing liquid at all times, as by the above arrangement of submergence, such flow being in a direction longitudinal of the path of the work surface and the turbulence being substantially uniform across the surface.

A chief critical factor is that the flow is turbulent as distinguished from the kind usually called streamlined or laminar, whether such turbulence is attained simply by selection of appropriately high velocity for the liquid as it travels from one end to the other of the tank traversed by the aluminum strip or like article, or whether it is aided by bafiies or other supplemental means of a kind conventional for promoting turbulence. It has been found that in the absence of the defined conditions, excessive dis solution or burning of the oxide films occurs as the current density is raised or the electrolyte concentration increased in efforts to achieve desirably thick films at rates of speed of the strip which would otherwise be high enough to make continuous anodizing economical. Indeed it now appears that it has been essentially impossible to attain such coatings in accordance with prior proposals for achieving continuous treatment by using high current densities; for any given strip speed and electrolyte concentration a limit has been found (usually corresponding to a relatively thin film) beyond with increase of the coulombic charge affords no further increase of film thickness whatever. In contrast, with the above-defined condition of turbulent electrolyte flow along the surfaces where anodic films are being formed, and with moderate electrolyte concentration, a plot of film thickness against current density slopes upward to very useful values.

The reasons for the effectiveness of turbulent electrolyte flow in obviating or inhibiting dissolution of the oxide coating are not fully understood. Although it is now believed that one function of turbulence is in aid of heat removal at the coating surface, it is also now considered that the chief source of heat in high-current anodizing is at the base rather than the surface of the oxide coating as it forms. The operation, moreover, does not seem to be critical with respect to the temperature of the circulating liquid, over a wide range; satisfactory results can usually be expected by keeping each body of electrolyte (e.g. as traversing the surfaces being anodized) at any point in the range, for example from room temperature (say, 20 C.) upwards. In any event, and regardless of theory, the described process has been found effective in producing anodic coatings as stated, e.g. even of the order of one mil thick, or in the range up to mils, a result not believed to have been obtained heretofore in a practical continuous operation.

Likewise in furtherance of one or more of the objects stated above, and for other ends as may become apparent, apparatus pursuant to the invention comprises tank structure having provision for continuous traversal of the elongated article through the electrolyte in the tank, preferably in a horizontal direction and very preferably through the ends of the tank below the electrolyte surface, e.g. through end seals. Electrode structure is provided, advantageously above and below the passing article, or at least in certain portions of the tank, whereby current flow follows a path from one electrode through the liquid to the article and from the article through the liquid to the other electrode. Where differential or one-side anodizing is required, means are included for isolating the portions of the liquid on opposite sides of the strip from each other, a special feature of the apparatus being the inclusion of separating elements removably associated with the side wall parts of the tank. In accordance with the invention, it has been found highly desirable to maintain the same electrolyte liquid on both sides of the strip, with the advantage of avoiding non-uniform results such as differences in the character of anodic film adjacent the edges, which might be caused by seepage or intermixing of electrolyte at the latter regions. With identity of electrolytes such problems are avoided.

The system further includes means for directing a continuing and rapid flow of electrolyte along the surface of the article, e.g. in a direction parallel to its direction of travel, whether concurrent or countercurrent with such flow. Such longitudinal flow along a strip is markedly superior to flow in a transverse direction, i.e. in affording distinctly more uniform film properties across the width of the strip. The tank is arranged, preferably by disposition and dimensioning of electrodes above and below the path of the strip or other article, so that flowing electrolyte is relatively confined, preferably in most cases to a rather shallow region along each surface, while as explained above the process contemplates that the liquid flow should be of turbulent character, as distinguished from laminar flow. While baffle means or the like may sometimes assist such turbulence, it will be appreciated that the latter can in many cases be achieved by appropriate selection of the flow rate, in accordance with known principles.

A greatly preferred embodiment of the apparatus includes a tank of double character, involving in effect two endwise abutted tank chambers having a closely restricted but not necessarily sealed passage between them for the moving article. Similar provision for electrolyte flow is achieved in each of these sections, preferably with electrolyte from separate sources moving from end distributing pipes to like receiving pipes at the partition between the tank sections, i.e. respectively from the opposite ends of the set of tank sections, so that one flow is concurrent and the other countercurrent respecting the strip travel. It has been discovered that the system and operation may involve supply of electrolyte from a common source to separate regions of the tank system, eg. upper and lower portions of the anodizing section, without short-circuiting, providing the piping that conducts the electrolyte to the inlet and outlet ends is of sufficient length and is suitably insulated electrically, so that long current paths are provided and thus electric current flow is negligible through the liquid in the piping.

In an especially advantageous form of the invention, the first tank section is circumstanced to provide cathodic cleaning of the passing article, while the second section is utilized for the anodizing operaton, which may, as indicated, be of two-side or one-side character as desired. Indeed a further feature is the adaptability of the described system to, or its combination with, appropriate switching or other circuit-shifting means whereby it can be employed for a variety of types of treatment and of current supply, including the novel operation of continuous differential anodizing, the latter being achieved, for example, by separate control of voltage or current characteristics with respect to opposite sides of a sheet under treatment.

Further features of the procedure and apparatus of the invention are set forth in the following description and in the accompanying drawings, wherein:

FIG. 1 is a perspective view of one embodiment of cell structure according to the invention, shown in simplified structural form and somewhat shortened in proportions, for clarity of illustration;

FIG. 2 is a longitudinal, fragmented, vertical section as if on line 22 of FIG. 1, somewhat enlarged relative to the latter;

FIG. 3 is a simplified vertical section on line 33 of FIG. 2;

FIG. 4 is a section similar to FIG. 3, showing a modified form of aperture between the cleaning and anodizing cells;

FIG. 5 is a vertical cross-section of one of the cell compartments of FIGS. 1 and 2, somewhat further enlarged, and taken on line 55 of FIG. 2;

FIG. 6 is a fragmentary horizontal view of one of the cell compartments, taken as if in section on line 66 of FIG. 5, i.e. at various levels as therein indicated, and with the strip under treatment omitted; and

FIG. 7 is a diagrammatic view of the apparatus, with the cells shown as if in longitudinal vertical section, including associated electrical and electrolyte-circulating arrangements.

As shown in FIG. 1, the illustrated form of apparatus embodies a cathodic cleaning cell compartment 10 and an anodizing cell compartment 11 through which the work continuously advances, e.g. a continuous sheet of aluminum 12, shown as entering the apparatus at the lefthand end and progressing through compartments 10 and 11 to move outward at the righthand end. With reference also to FIGS. 2, 3, 5 and 6, the cell 10 consists essentially of an upper plate 14 and a lower plate 15, respectively constituting electrodes and arranged in parallel coextensive relation and separated by insulating side walls 16, 17. Similarly the anodizing compartment 11 is enclosed by upper and lower electrode plates 18, 19 separated by side walls 20, 21. In all respects here material the two compartments are similarly constructed, i.e. as to the central regions where electrolytic action occurs, so that illustration and description of one may be taken to characterize the other as well. In the example of apparatus shown, electrode plates 14, 15 and 19 are made of lead and the plate 18 of stainless steel, so that the latter may be thinner, as indicated, but this difference does not affect the principles of construction of the equip-- ment. It will also be appreciated that mechanical details, as of fastening means for various parts, reinforcing structure for some or all of the electrode plates, mounting structure, extended electrical contact means over the surfaces of the electrodes, and the like, are omitted from the drawings for clarity, being unnecessary for understanding of the invention.

As will be seen, the strip 12 is advanced longitudinally through the compartments and 11 in succession, passing in a horizontal plane midway between the electrode plates 14, 15, and 18, 19, the cell compartments being filled with liquid (aqueous) electrolyte so that both surfaces of the strip are in submerged contact with the liquid, and the electrodes, parallel to the strip, are at least approximately coextensive in area (or of greater area) with respect to the opposite surfaces of the strip in each compartment. The path of the strip 12 is thus in dicated by the dot-and-dash line 22 in FIG. 2. It will be understood that suitable means, not shown, may be provided for advancing the strip or other elongated article under treatment, as by winding it on a take-up reel beyond the righthand end of FIGS. 1 and 2, or indeed beyond further treating equipment such as for Washing and drying, the strip thus being unwound from a suitable payoff reel to the left of the apparatus as shown, such take-up and pay-ofi equipment being of conventional character and therefore not illustrated.

Means are provided for circulating electrolyte continuously along both sides of the strip in each of the compartments. Thus, for example (FIGS. 1 and 2), a length of pipe 25 of insulating material is cut away along one side at 26, so as 0 open sidewise into the entering end of the compartment 10, being attached thereto with suitable connecting members as generally indicated at 27, 28 for the respective electrode plates 14 and 15, such connecting structure including transverse parts or strips of insulating material 29, 30 cemented to the edges of the pipe opening. Likewise a pair of insulating members 31, 32 are cemented or otherwise fastened to the edges of a similar opening along the other side of the pipe section 25, providing a tapering entrance for the sheet 12, and faced by an end plate 34 which has a horizontal 6 to the other edges of the pipe segments 51, 52 afford a flaring seal between the pipe segments and the entrance ends of the electrode plates 18, 19, being secured by appropriate fastening means as indicated at 59, 60. At one lateral end, the pipe segments 51, 52 and other parts of the double distributing chamber are closed as by the plugs 61 and associated wall structure 62 generally indicated in FIG. 1. At the other end, these pipes 51, 52 open into connecting pipes 53, 64-. Hence suitable compartment or chamber is thus afforded at the end of the cell 11, having in effect two portions extending across the cell and respectively connected for electrolyte circulation with the pipes 63, 64. At the remote end of the anodizing compartment 11, a like double distributing chamber is afforded, with the pipe segments 67, 68 of insulating material, cut away respectively along lower and upper sides and joined to the electrode plates 18, 19 by insulating plate-like structures 69, 70 whereby the space of the cell communicates in upwardly and downwardly flaring relation with the end chamber. Similar parts 71, 72 secured along the other edges of the pipe segments 67, 68 aflFord a flaring wall for the opposite crosswise extending side of the end chamber, where the strip exits through a slot 74- in an end plate 75 As in the other similar structures, one lateral end of the pipe segments and the distributing chamber is closed by plugs 77, 78 and Wall structure 79, while the opposite ends of the pipe segments 1 67, 68 communicate with connecting pipes 81, 82.

slot 35 through which the sheet enters. Thus the described structure aifords a liquid distributing head or chamber at the entrance end of the cell compartment 10, extending across the latter, and appropriately closed at one end as by a plug 36. At the other end, this compartment, by appropriate flange connection 37, communicates with a further pipe 38, likewise preferably of insulating material. At the strip exit end of the cell compartment 10 a similar distributing chamber is afforded by a like pipe section 40 extending across the apparatus, cut away at one side 41 and connected to the plates 14, 15 of the cell by similar means 42, 43. Along the other side of the pipe section 40, appropriate insulating members 44, 45 provide a tapering exit for the strip, Where it passes into a central aperture plate 46 having a long horizontal slot 48 through which the strip passes. The distributing chamber is appropriately closed at one lateral end as by parts including the plug 49 and shown in FIG. 1, and at the other end communicates with a connecting pipe 50.

With respect to the cell compartment 11, where means for isolating or partitioning upper and lower regions, by the strip, may be provided if desired, electrolyte distributing means are atforded both above and below the path of strip travel at each end. Thus a pair of pipe sections 51, 52 of insulating material are arranged at the entering end of the compartment, one above and one below the locality of the strip under treatment, and respectively cut away along their lower and upper sides at 53, 54. Insulating members 55, 56, extending across the apparatus and cemented or otherwise secured to one edge of the open regions of the pipe members 51, 52 provide a tapering communication to the central slot 48. Likewise, appropriate plate-like structures 57, 58, secured Provision is also made for inhibiting or preventing escape of electrolyte from the end slots 35, 74 where the strip enters and leaves. For instance, one appropriate sealing arrangement for this apparatus is shown at the righthand end of FIGS. 1 and 2 and comprises a box 84 extending across the apparatus, being attached to the end plate 75. The box 84 has a horizontal exit slot 85 at its outer side, in registration with the slot 74, and contains two elongated bodies of sponge rubber 86, 87 arranged to have long plane surfaces respectively in contact with upper and lower surfaces of the strip 12, which thus passes between the sponge rubber bodies. The pressure plate 88, of suitable insulating material, extends across the uppermost compressible body 86 and is engaged by turn screws 89 threaded in appropriate means, such as the upper wall of the box, whereby the sealing assembly of the sponge rubber bodies may be vertically compressed for the necessary sealing engagement with the strip. An identical end seal is provided at the strip entrance of the apparatus, generally designated 90, and need not be further described in detail.

As shown in the drawings, especially FIGS. 5 and 6, the side wall members 20, 21 of the anodizing compartment 11 are sealed to the upper and lower electrode plates 18, 19 by longitudinal rods or strips of resilient material such as rubber or Neoprene 92, partly resting in corre sponding recesses or channels in the upper and lower faces of the side blocks. Suitable means are provided for securing electrode plates and side blocks or Walls 20, 21 together, as for example the bolts 93, 94 which may be insulated from the electrodes by flanged bushings 95 of appropriate insulating material. Slotted sheets of plastic material 97, 98 are arranged respectively at the lower face of the upper electrode 18 and at the upper face of the lower electrode 19, being appropriately secured to such faces, for avoidance of contact of the strip 12 with the electrode faces should severe deflection of the strip occur as it transverses the cell compartment. As shown in FIG. 6 these sheets, there exemplified by the sheet 93, have a series or array of very large openings or slots 99 o ver by far the major part of their horizontal area, so that there is relatively little obstruction to the full contact of the corresponding electrode plate with the electrolyte. For clarity, the structure shown in FIG. 6 is somewhat shortened in the direction crosswise of the strip path, it being understood that the width of the apparatus is in any event designed to accommodate the dimensions of sheet or strip to be treated. The structure and parts just described with respect to the anodizing compartment 11 are also conveniently embodied in the cathodic cleaning compartment 10, i.e. for securing the plates to the side walls and for preventing contact between sheet and plate, and need not be separately shown and explained.

The cell compartments may also include means, in the nature of side rails or guides for loosely engaging the longitudinal edges of the sheet under treatment, especially for isolating upper and lower portions of the anodizing compartment, although also, if desired, for restricting vertical or transverse movement of the strip in traveling through each compartment. Thus as shown in FIGS. and 6, and also indicated in FIG. 2, a pair of such side rails 100, 101 each consisting of an elongated block or member are disposed longitudinally of the anodizing compartment at the lateral sides of its interior. These members 100, 101 are slidably dovetailed into the respective side wall members 21, at 102, 103, i.e. so that on disassembling the equipment the side rails can be fitted into the side elements, or removed therefrom as desired. As shown, they are conveniently shaped to have plane surfaces facing the central region of the compartment, and may also be appropriately tapered in a lateral direction to the dovetailed region of each. In the centrally facing surface of each there is a groove, as indicated at 104, 105 shaped to engage the passing strip with some looseness, i.e. so that there is substantial separation between the upper and lower zones but little or no frictional bearing on the strip. These side rails may, if desired, conveniently extend into the liquid distributing compartments, especially compartments 51-52 and 67-68 of the anodizing cell, being backed up laterally by appropriate blocks as indicated at 106, 107 in FIG. 6, whereby the isolation of the two cell compartments extends into the end regions. Such disposition of the side rails is indicated by dot-and-dash lines outlining the rail 100 in FIG. 2. Similar side rail structure may be provided for the cleaning cell compartment, although a separate illustration of such arrangement is omitted as being duplication of the showing in FIGS. 5 and 6. As will be appreciated, a pair of removable side rails (or two pairs of same if desired for both compartments) should be provided for each width of sheet to be treated in the apparatus, the pair of rails being such as to accommodate the strip with a slight overlap, say one-eighth inch, at each side whereby the sheet or strip traverses the grooves 104, 105. For example, a groove one-eighth inch wide (i.e. in the vertical dimension as seen in FIG. 5) and one-fourth inch deep is suitable for accommodating a strip penetrating such groove by one-eighth inch, and having a thickness from that of foil up to a little less than one-eighth inch. In similar fashion the slots 35, 4S and 74 (FIG. 2) in the end and center plates of the assembly may, for example, have a width (vertical dimension) of one-eighth inch to accommodate like thicknesses of strip, these slots being conveniently faced, i.e. over the surfaces facing the strip, with sheeting of suitable plastic material having low friction properties.

Although the apparatus shown is designed for handling strip material, whether foil or thicker strip, herein sometimes generically defined as sheet, it may equally be adapted for other shapes of continuous elongated articles, such as wire or rod. Thus whereas FIG. 3 schematically illustrates the slot configuration 48 at the central barrier for sheet 12, FIG. 4 shows an alternative arrangement of similar parts designated 44a, 45a, 49a and 50a where the central barrier opening is a small cylindrical hole 48a to accommodate wire or rod. It will be understood that slots at other localities of the apparatus are then similarly shaped and arranged, and that other structures may be provided such as with a plurality of such holes to take a number of wires at once, or there may be openings of yet further shape for specific configurations of the passing article. The materials of construction of the apparatus are appropriately selected for locality and service, with suitable properties of electrical insulation, strength and resistance to acid electrolyte, to the extent necessary. Thus, for example, the pipe segments 25, 40, 51, 52, 67 and 68, and likewise associated members and center and end plates constituting the liquid distributing assemblies are made of strong insulating composition, as likewise the side Walls or seals 16, 17, 2t) and 21 and indeed other enclosure parts including side closures, end seal box 84 and similar structure of end seal 90, the removable side rails such as shown at 100, 101, and plugs for the pipe segments as at 36, 49, 61 and 77, 78. Appropriate materials for these elements are fiber filled resin bodies of acid resistant nature, e.g. asbestos filled or textile filled resins, the former being particularly suitable for pipes and heavy structures required to have considerable strength. Pipe, plate, and other forms of such material are readily available, for shaping and assembly into structures of the sort shown. In general the arrangement is such as to effectively insulate the electrode plates 14, 15, 18 and 19 and to avoid exposure of other metallic surfaces to the electrolyte in or around either compartment. As will be appreciated, means are also provided for electrical contact with the electrodes, such being here simply illustrated as the several lugs 108 and heavy braided conductors 109 in FIG. 1, further parts secured to the electrode plates being used if necessary (not shown) for greater area of electrical contact.

Although the dimensions and proportions of the cell may vary widely to suit requirements of use, it is ordinarily desirable to have the electrode plates closely spaced so that the electrolyte filled cell region above and below the strip or the like is relatively narrow in a vertical direction. In one example of apparatus according to the invention, the electrode plates are all 60 inches long in the direction of strip travel and have a width to provide an internal cell space, i.e. inside the members 20, 21, or 16, 17, of about 21 inches. Hence the electrolytically functioning regions of the two cell compartments are each about 5 feet long and can accommodate strip up to about 20 inches wide. In the described embodiment, the side members 16, 17, 20 and 21 are approximately 2 inches square in cross-section, whereby there is about one inch of space above and below the sheet in each compartment. As will now be seen, the strip 12 continuously travels through the assembly, entering the seal 90, crossing the first distributing chamber 25, traversing the cleaning cell compartment 10 and the liquid distributing chamber 40, after which the strip traverses the central slot 48, crosses the double distributing chamber 51-52, passes along centrally of the anodizing compartment 11 and after traversing the further double liquid chamber 67-68, leaves, via the end seal in the box 84. Liquid electrolyte is circulated continuously through both compartments, both above and below the strip, e.g. in a condition of turbulent flow as explained above. In general in most cases, the direction of flow may be either countercurrent or concurrent to the actual direction of strip advance, although for some purposes countercurrent liquid flow is presently preferred for the anodizing cell.

Referring to FIG. 7, where the parts are shown in diagrammatic fashion and the initial portions of connecting pipes for the liquid distributing chambers, as at 38, 50, 63, 64, 82 and 81 are indicated in dot-and-dash lines (with certain of these depicted in distorted form, for illustration purposes), suitable circulation and temperature control means generally designated 110 and 111 are provided respectively for the electrolyte flows in the compartments 10, 11. These may be of any appropriate nature and combination of components designed to advance the electrolyte continuously and control its temperature as with the aid of suitable cooling, and may be conveniently identical with each other. One set of such components is shown for the system 111 in FIG. 7, with the understanding that the system 110 may be the same, except that its heat exchange capacity, for cathodic cleaning, can be less than for the anodizing section. Thus by an appropriate pipe 115 connected with the pipes 63, 64, liquid leaving the upper and lower zones of the anodizing compartment 11 is received in a suitable reservoir 116 and from the latter advanced by a pump 117 through a heat exchanger 118 to return to the other end of the cell via pipe 119, i.e. to the inlet distribution chamber 67-68. The heat exchanger may be of conventional tubular design, arranged for cooling of the electrolyte by fiow of water or other coolant through inlet and outlet pipes 120, 121, and may be controlled by a suitable temperature regulator, e.g. a commercially available recording temperature controller 122, having a tempera ture sensitive element 123 in the electrolyte pipe 119 and operating a suitable motorized valve in the coolant flow line 120. Thus the function of the described equipment in the circulation system 111 is to advance and control the temperature of the electrolyte, eg by removing heat from it so as to maintain a substantially constant temperature therein. The pump 117 appropriately propels the electrolyte at a sufficient rate to achieve the desired turbulent flow above and below the strip.

A particular feature, indeed adaptable to other combinations of electrolyte circulation with other or greater numbers of cell compartments or regions, is the arrangement of piping connections between the distribution heads or chambers and the circulating means. Thus each of the pipes 63, 64 extends through continuations thereof (of the same diameter) 125 and 126 respectively, joining a T connection 124 to the pipe 115. It is found that by selecting piping of suitable length and diameter from the upper and lower parts of the distribution chamber 51, 52, there can be complete avoidance of appreciable electric current flow or leakage between these two chambers even though the electrodes 18, 19 may be operated at different electrical potentials or indeed at opposite electrical polarity. A similar arrangement is provided for the pipes 82, 81, i.e. by extensions of such pipes as shown at 128, 127 to a single T connection 129 for the outlet pipe 119 of the circulation means. In other words, where the operations on the upper and lower surfaces of the strip 12 traversing the compartment 11 are such as to differ electrically or otherwise to require electrical isolation of the electrolytes, the supply of electrolyte to both zones may come from a common system, by utilization of the described arrangement of piping, i.e. the pipes of insulating material. As will now be appreciated, the length of the connecting pipes between the distribution chambers and the T joints 124, 129 are readily determinable from the diameter of the pipe, the specific conductance of the electrolyte and the maximum voltage difference between the upper and lower electrodes 18, 19. Where the pipes had a 3 inch inside diameter, the electrolyte was a 15% solution of sulfuric acid .and the voltage between the electrodes was about 30 volts, it was found that pipe lengths of 8 feet in each leg were sufiicient to reduce leakage current through the electrolyte to a negligible value. As will be seen, the electrolyte is advanced countercurrently to the strip in the anodizing cham ber 11 and concurrently with the strip in the second chamber 10, leaving the compartments at the center of the apparatus through the dual chamber 51-52 and the single chamber 40 respectively.

As indicated above, the flow through the compartments 10, 11, most especially through the anodizing compartment 11, is turbulent in nature. This can be achieved by sufiicient rapidity of flow under the circumstances of the flow path, including its cross-section between the adjacent electrode and the strip, the rate being thus selected so that the liquid travels in turbulent, rather than laminar fashion in accordance with established principles of liquid flow in a conduit filled thereby. As stated, this feature of maintaining turbulence in the electrolyte as it passes the strip, whether by selection of appropriately high velocity or with the aid of baflles or other supplemental means (not shown) is found important for effective results in high speed, continuous anodizing to achieve porous-type anodic coatings, especially of thicker character, at high current densities, e.g. upwards of amperes per square foot of exposed strip surface. In general, although conventional auxiliary means may be used, turbulence is satisfactorily achieved by a sufliciently rapid rate of flow. By providing such turbulence throughout its extent across the strip, efiicient heat removal is easily attained to permit maintenance of desired temperature values, with suificient volume of electrolyte per unit time as to have no more than an insignificant temperature rise (eg one or two degrees) between the localities of liquid inlet and discharge of the cell compartment.

As will be understood, ascertainment of suitable conditions, for instance a sufiiciently high rate of flow, to provide turbulence is generally a matter of calculation under known principles, as by determination of flow rate of the aqueous electrolyte, for the given cross-sectional shape and size of path, which will yield a Reynolds number in the range of turbulent flow. Thus conventionally such numbers larger than about 2500 signify a condition of turbulence, it being understood that for best results in the present process, the flow conditions should be characterized by a substantially higher Reynolds number than the minimum. For example, a liquid flow affording a number of 20,000 is satisfactorily efficient for anodizing operation at a current density of 600 amperes per square foot, and conditions of even greater turbulence, e.g represented by Reynolds numbers as high as 100,000, can well be used, as for very high current densities. Moreover, in many cases, determination of a suitable flow rate for achieving turbulence and for maintaining a desired temperature in the passing liquid without more than a few degrees of rise can be made by simple preliminary tests and observations.

FIG. 7 also illustrates one system of electrical connections for the apparatus, including switching means (which may be replaced by other, equivalent shiftable connection devices if desired) whereby a variety of different types of operation may be selectabl achieved. Thus the electrodes 14, 15 of the cleaning compartment 10 are connected together and to one contact of a switch 131 by conductors 132 and 133, the switch .arm 131 being in turn connected to the positive terminal of a current source 135, i.e. a DC. source. One electrode, for instance, the upper electrode 18 of the anodizing cell 11, is connected via a conductor 136 to the other terminal of the DC. source 135. The other electrode, eg the lower electrode 19 of the cell 11 is connected to a switch 138 having four contacts of which the first, 139, is connected to the electrodes 14, 15 of the cleaning cell and thus in eifect to the positive terminal of the source when the switch 131 is closed to its contact 130. A second contact 140 of the switch 138 is connected to a second contact 142 of the switch 131. A third contact 143 of the switch 138 is connected to the conductor 136, i.e. thus to the negative terminal of the current source. Finally, a fourth contact 144 of the switch 138 is connected through a resistor 145 to the conductor 136 and via the latter, to the negative source terminal.

With the stated switching arrangement, or equivalent means, the system is flexibly adapted to a variety of types of treatment. For example, if switch 131 is closed to contact 130 and switch 138 is closed to contact 143 the operation of continuously cleaning and continuously anodizing both sides of the strip may be performed. Electrodes 14 and 15 are both connected to the positive terminal of the source, while both electrodes 18 and 19 are connected to the negative terminal, current thereby flowing from the first-named electrodes through the electrolyte bodies in the compartment 10 to the strip, for cathodically cleaning both surfaces, such current then passing through the strip and traveling into and through the electrolyte bodies (with the strip as anode) in compartment 11, to the electrodes 18, 19, thence returning to the negative terminal. If switch 138 is turned to its contact 144 instead of contact 143, again two-side cleaning and anodizing is effected (switch 131 remaining as before), but the operation is of differential character in the compartment 11. The resistance 145 reduces the current flow in the lower half of the cell, so that for example a thinner coating is applied on the lower surface than on the upper surface. Conveniently, the resistor 145 may be adjustable and may assume any suitable character (for instance, an electrolytic resistor comprising a column of sulfuric acid solution with movable lead electrodes), its function being a voltage dropping device and being indeed attainable in a variety of ways. In fact, if the cell system is appropriately designed the necessary voltage drop for differential anodizing can be achieved by corresponding differences in spacing of the electrodes from the strip.

If it is only necessary or desirable to apply the anodic film to one side of the strip, certain other connections are used. One such arrangement is to maintain switch 131 connected to its contact 130, while shifting switch 138 to contact 140. In this scheme, cathodic cleaning is effected on both sides of the strip, while only the upper electrode 18 is energized in the anodizing compartment, current thereby traversing only the upper body of the electrolyte in the latter and building a suitable film on only the upper surface. In such case, as indeed in all cases of one-side anodizing and likewise differential anodizing, the side rails are appropriately installed, at least the side rails 100, 101 of the anodizing compartment. Another and particularly satisfactory arrangement is to connect the switch arm 138 to its contact 139, while switch ar-m 131 remains in contact with its point 130. Here not only the electrodes 14 and 15 of the cleaning compartment but also the lower electrode 19 of the anodizing compartment 11 are connected to the positive terminal of the source, so that some of the anodizing current between the strip and electrode 18 has traveled along the strip from the compartment 10 while the remainder has traversed the lower body of electrolyte in compartment 11 and entered the lower face of the strip. In this way a relatively higher current density may be achieved or less heating of the strip by current flow, than in the firstmentioned arrangement for one-side treatment.

A still further set of connections for one-side operation is achieved by setting switch arm 131 on its contact 142 and switch arm 138 on contact 140. In this case there is no cathodic cleaning and the DC. source is directly connected to the electrodes 19 and '18 with the former positive and the latter negative. Anodizing current then simply flows from the electrode 19 through the lower body of electrolyte to the strip and from the strip to the upper body through the electrode 18, building up the film on the upper surface of the strip. It may incidentally be noted that the apparatus is adapted for alternating current operation without cathodic cleaning, as by connecting one terminal of such source to the electrodes 14, and the other terminal to the electrodes 18, 19, or alternatively to single electrodes on only one side of the strip for oneside A.C. treatment. Graphite electrodes are preferably used, rather than lead, for A.C. operation.

Even though the central slot 48 and the grooves in the side rails or guides, such as grooves 104, 105 have a slightly or somewhat loose fit with the passing article, suflicient isolation of electrolyte bodies, both electrically and otherwise is ordinarily achievable so that the desired variety of operations can be attained. Indeed electrolytes at different flow rates, with different temperatures maintained in them, can be utilized as between the cathodic cleaning and anodizing compartments. Moreover, with the side rails in use there is insufficient electrical leakage in the anodizing compartment to adversely affect one-side anodizing treatment or differential treatment. In general, it is greatly preferred to utilize liquids of the same composition in the upper and lower parts of the anodizing chamber even where these parts are isolated. In such fashion, slight seepage of the electrolyte around the edges does not then afford any undesirable modification of the film on one side or the other adjacent such edges of the passing sheet. With other forms of apparatus, it is conceivable that the process, used for anodizing one side alone, may employ other aqueous liquids than an anodizing electrolyte for the body .adjacent one surface of the strip, in situations where no current is to flow through liquid to the strip at such locality. However, it is greatly preferred to use a liquid electrolyte, exg. of the same composition, even under such circumstances. It may also be noted that the center plate 46 need be no more than of moderate thickness, say one inch, and particularly that the arrangement of the distribution chambers 40 and 51-52 is such as to afford a turbulent state of the elec trolyte essentially up to the entrance and exit of the slot 48. Under such circumstances, where a high electrical current may be traveling through the strip, heating effects are effectively minimized at this locality.

In performance of the process, as with apparatus of the character shown, the speed of strip travel is generally governed by the thickness of anodic film desired, having regard to the length of the anodizing path and the applied current density. For many purposes, speeds of the order of l to 20 linear feet per minute are suitable, in cells, such as cell compartment 11, from 1 to 5 feet long and with current densities at the surface or surfaces under treatment, in the range of amperes per square foot and above, more usually upwards of 250 amperes per square foot, and indeed preferably of the order of 500 to 1000 amperes per square foot. Thus, for instance, anodic films on sheet, up to and including 1.0 mil in thickness on the sheet surfaces have been produced with high efficiency using an aqueous electrolyte of 15% sulfuric acid maintained at approximately 65 C. in the bodies on both sides of the strip, with current densities of 250 to 1000 amperes per square foot and at convenient rates of strip travel. In such case the electrolyte flow rate was about 5 linear feet per second, achieving abundant turbulence, uniformly across the strip. Thus in an anodizing tank five feet long, at a current density of 650 amperes per square foot, 1.0 mil films are produced with a strip speed of 5 linear feet per minute and films of 0.2 mil thickness at a corespondingly faster speed of 25 feet per minute, very thin films (of the order of 0.01 mil) being attainable at high strip speeds, e.g. 250 to 750 feet per minute.

The electrolyte to be employed, especially in the anodizing compartment, is selected in accordance with required operation, it being found that exceptionally satisfactory results are obtained with sulfuric acid electrolytes, i.e. aqueous solutions of sulfuric acid. In general, electrolyte concentrations below 2% sulfuric acid are too low, and develop excessive heat by their resistance to current travel. Concentrations above 50% generally appear to have too much dissolving power on the oxide film, for attainment of thick coatings. A more useful acid concentration is 5% or preferably 10% or more, present practice of the invention being to employ 15% acid electrolytes. Ordinarily it appears unnecessary to use concentrations above 40%. In general, of course, rapidity of action is enhanced at higher concentrations but these involve greater limitations on the film thickness and tend to require very short times for anodizing and correspondingly very high and sometimes impractical current densities. References herein to acid or other concentrations in solution will, of course, be understood to mean percentages by weight.

In the cathodic cleaning compartment 10, the electrolyte may be such as suitable for this purpose. Quite conveniently sulfuric acid is effective here, and indeed acid solutions of concentrations similar to those preferred for the anodizing cell. If there is substantial difference in composition of electrolyte between the compartments 10 and 11, greater care may be required to avoid mixing at the vicinity of the connecting orifice, as for example by inclusion of a short rinsing section of tank (not shown) with orifices at its ends. Where the two electrolytes are of the same composition, as is now preferred, there is of course no problem at all about mixing. Indeed the electrolyte temperatures maintained in the two compartments can be different if desired, and likewise the treatment times, as by constructing the compartments to have different lengths. The cathodic cleaning action is found to be very effective in the illustrated apparatus, being both electrochemical and chemical. Hydrogen is usually evolved from the surface of the metal in this compartment, and such evolution, combined with the chemical action of the electrolyte, cleans the surfaces of extraneous foreign matter such as dirt and grease. Although sulfuric acid electrolytes are described above for the procedure of the invention, other electrolytes appropriate for anodizing operations may be employed in some cases, e.g. acid electrolytes of generally equivalent function. Thus examples of suitable electrolytes for the anodic treatments here described are sulfuric acid, chromic acid, diand tri-basic organic acids, or their equivalents, either separately or in suitable combination, all as will be readily understood by persons familiar with the art of anodic treatment of aluminum to produce porous-type oxide films.- It will be understood that if the apparatus is used for treatments, e.g. in the compartment 11, of different nature than the described production of porous oxide films, as for example to effectuate electrolytic brightening, electrolytic polishing or other film operations where the metal is connected as anode, appropriate electrolyte compositions are employed, in accordance with known principles of the art relating to such treatments.

In practice, the temperature maintained in the electrolytes may be selected generally in accordance with known principles, a presently understood advantage of the turbulent flow conditions being that satisfactory heat removal is achieved despite the present belief that outer portions of the oxide film under formation may tend to impede heat transfer from the base of the film, and despite the large heating effect of high current densities. Thus in the anodizing compartment, temperatures within a wide range, even up to about 100 C., may be selected although it is distinctly less practical to attempt maintenance of values below 20 C. The range of 40 C. and upwards seems presently quite feasible. Similar temperatures appear useful in the cathodic cleaning operation, i.e. for the electrolyte, and indeed it may be convenient to utilize-acid solution at the same selected temperature; in the latter event, it is conceived that a single circulating and temperature control system for electrolyte in all parts of the apparatus could sometimes be used as explained above. Alternatively, rather higher temperatures may be preferred for the cathodic cleaning. Thus, for instance, with a 15% sulfuric acid electrolyte kept at approximately 80 C., and current densities of 1000 amperes per square foot of sheet surface area, remarkably complete cathodic cleaning has been attained in a tank feet long (as shown) with strip speeds up to 750 feet per minute. The contact time was thus only 0.4 second, while by comparison, utilizing the same electrolyte at the same temperature but without any applied current, comparable surface conditions could only be attained in at least about one minute of exposure.

As will now be appreciated, current densities, notably for the anodizing operation, may vary over a large range, e.g. from 100 to 4000 amperes per square foot of surface being anodized. The invention is particularly significant at values of 300 amperes per square foot and notably above, for achieving rapid anodizing action without sacrifice of film thickness and character. Indeed values of current density of 600 amperes per square foot and upwards, e.g. to 1500 amperes, represent a special range of departure from anything heretofore deemed practical. Above 4000 amperes per square foot (and to some extent above 2000), it appears that with ordinary mechanical arrangements for circulating electrolyte the desired heat removal tends to become unduly difficult.

The novel process of differential anodizing, readily achieved with apparatus of the sort shown whereby different electrical conditions, e.g. different current densities, are provided respecting opposite surfaces of the strip 12 in the compartment 11 (with switch 138 on its contact 144), has distinct advantages. For example, it enables the production of sheet or other articles having quite different anodic film thicknesses on opposite surfaces. Such products are conceived to be useful, for instance, in architectural or other building applications where a high measure of corrosion resistance is desired on one side of the sheet, with a relatively thick coating, while a lower degree of resistance is satisfactory on the other side. Presently for this purpose, the reverse side of the sheet is simply left bare, lacking any significant enhancement of its corrosion resistance. Thus this new product affords economic benefits over conventional twoside anodizing, i.e. in the saving of some current and other costs necessary for treating one of the sides, and also affords distinct improvement in respect to durability as contrasted with articles having only bare metal on such side.

Although a variety of specific conditions for various anodizing treatments have been set forth hereinabove, a further specific example may be given, as for the operation of the apparatus to apply like coatings on both surfaces of a continuing strip 12 of aluminum. In such operation, the electrolyte in both of the cell compartments 10 and 11 was a 15 sulfuric acid solution, and the temperature control was exercised to keep the turbulently traveling acid solution at about 65 C. in each compartment, there being a rise of not more than about 1 or 2 C. between the compartment ends in any case. Each of the systems and 111 provided a total flow rate of about 300 US. gallons per minute, i.e. approximately 150 gallons per minute along each surface of the strip in the cleaning tank 10 and likewise in the anodizing tank 18. With the cross-section of the flow passage adjacent each surface of the strip having dimensions of about 1 inch by 20 inches, such electrolyte flow was attained at a linear flow rate of 2.5 feet per second. With a current density of 650 amperes per square foot, i.e. at the exposed upper and lower surfaces of the strip 12 in the anodizing compartment 18 (the switches 131 and 138 being set at their contacts and 143 respectively), highly satisfactory porous-type anodic coatings were achieved. Each of the tanks was 5 feet long and strip speeds of 5 feet per minute yielded films of 1.0 mil thickness, while at 25 feet per minute a thickness of 0.2 mil was readily attained. According to tests, as above, linear electrolyte flow velocities of about 2.5 feet per second or more appear completely adequate and afford preferred efficiency, but in a more general sense, liquid velocities of about one foot per second and above can be considered useful.

In the treatment of wire, for instance as mentioned above relative to FIG. 4, very good results have been realized, yielding good electrical insulation characteristics of the anodic coating and high scrape abrasion resistance, at speeds of wire travel up to 200 feet per minute.

The efficiency of the process, for instance in operation under presently preferred conditions as exemplified above, seems unusually high. The coulombic input found necessary (in pilot plant operation) to produce an oxide film of 1 mil thickness, utilizing the described process is about 39,000 coulombs per square foot, this representing about 90% coulomb efficiency in contrast to about 70% efficiency (54,000 coulombs per square foot) in a number of prior operations for production of comparable films.

As indicated, the procedure and apparatus in some generic aspects are applicable to AC. anodizing operation. Likewise the cleaning treatment in tank 10 may be an AC. electrolytic cleaning, e.g. using electrolytes known for such purposes; in that event, appropriate electrical connections are employed, including a separate power source for tank 11, being A.C. for two-side and DC. for oneside anodizing.

It is to be understood that the invention is not limited to the specific operations, compositions and structures herein described but may be carried out in other ways without departure from its spirit.

We claim:

1. A method of continuously treating the opposite surfaces of an elongated aluminum article to produce an anodic coating thereon, comprising advancing said article lengthwise first through a region of submerged contact of said surfaces with cleaning electrolyte and continuously thereafter through a region of submerged contact of said surfaces with anodizing electrolyte which produces a porous coating on aluminum, said article being maintained in submerged position and in substantially continuous contact with electrolyte, beneath the upper parts of said regions during advance between them, while passing electric current through the cleaning electrolyte to the article as cathode in the first region and while passing electric current from the article, as anode, into the anodizing electrolyte in the second region, said current in the second region being passed at a density, of at least 100 amperes per square foot of said surfaces exposed in said second region, for producing, in coaction with said anodizing electrolyte, a porous anodic coating on the surfaces, and maintaining substantially constant temperature conditions in the electrolyte bodies adjacent the article surfaces in the said regions as the article advances, by turbulently advancing the electrolyte in each region over the surfaces in a direction lengthwise of the path of the article, and recirculating the electrolyte from each region through heat exchange paths for temperature control thereof.

2. A method as defined in claim 1, wherein the electrolytes are of identical composition in both regions and the anodizing current in the second region is passed at a density of at least 250 amperes per square foot of said surface exposed therein.

3. A method of continuously treating the surfaces of an aluminum sheet to produce anodic coatings on said surfaces, comprising advancing the sheet lengthwise first through a region of submerged contact of both said surfaces with cleaning electrolyte and thereafter continuously through a region of submerged contact of said surfaces with anodizing electrolyte Which produces a porous coating on aluminum, said article being maintained in submerged position and in substantially continuous contact with electrolyte, beneath the upper parts of said regions during advance between them, while passing electric current through the cleaning electrolyte on both sides of the sheet to both surfaces thereof, said article serving as cathode in said first region, and while passing electric current from the article into the anodizing electrolyte at both sides thereof in the second region, said sheet serving as anode therein and said last-mentioned current being passed at a density, of at least 100 amperes per square foot of the surfaces exposed in said second region, for producing, in coaction with said anodizing electrolyte, a porous anodic coating on each of said surfaces having a thickness of at least 0.1 mil, and maintaining substantially constant temperature conditions in both the cleaning and anodizing electrolytes adjacent the surfaces as the sheet moves through said regions, by turbulently advancing electrolyte over each surface in each region in a direction lengthwise of the path of the sheet and recirculating electrolyte from both sides of the sheet in each region through heat exchange paths for temperature control thereof.

4. A method as defined in claim 3, wherein the electrolytes are sulfuric acid solutions of identical concentration, selected in the range of 2% to 50%, in both regions and on both sides of the sheet in each region.

5. A method of continuously anodizing the surfaces of an aluminum sheet, comprising advancing the sheet lengthwise through a first region of submerged contact of both said surfaces with bodies of aqueous liquid at least one of which is an electrolyte and then through a second region of submerged contact of both said surfaces with anodizing, aqueous electrolytes that are mutually of the same composition which produces a porous anodic coating on aluminum, while passing electric current through electrolyte in the first region to the sheet and then through the sheet and then from the sheet into and through the anodizing electrolyte at both sides thereof in said second region, said sheet serving as anode in said second region and said current being passed at a density, of at least 250 amperes per square foot of the surfaces exposed in said second region, for producing, in coaction with the anodizing electrolyte, a porous anodic coating on each of said surfaces having a thickness of at least 0.1 mil, and maintaining substantially constant temperature conditions in said bodies of liquid in the first region and in said anodizing electrolytes, adjacent the surfaces as the sheet moves through said regions, by turbulently advancing each of said bodies of liquid in the first region and said anodizing electrolytes in the second region over each surface in the respective regions in a direction lengthwise of the path of the sheet and recirculating said bodies of liquid and said anodizing electrolytes from both sides of the sheet through heat exchange paths for temperature control thereof, said sheet being maintained in substantially continuous submerged contact with aqueous electrolyte while being advanced from said first region to said second region in continuously submerged position beneath the upper parts of said regions.

6. A method as defined in claim 5, wherein the electrolyte is a sulfuric acid solution having a concentration selected in the range of 2% to 50%.

7. A method of continuously anodizing both surfaces of an aluminum sheet, comprising advancing the sheet in submerged contact with bodies of anodizing electrolyte of like composition at both surfaces of the sheet, while passing electric current between the sheet and both said bodies of electrolyte for anodizing the surfaces in contact therewith, said current being passed at a density of at least amperes per square foot of each surface and being effective in coaction with the electrolyte to produce a porous anodic coating on each surface, continuously advancing both of said bodies of electrolyte in turbulent flow over the respective surfaces of the sheet in a direction aligned with the path of travel of the sheet, said flow being maintained in turbulence substantially uniformly across the sheet, and recirculating said bodies of electrolyte while removing heat therefrom, the aforesaid method being -a method for differentially anodizing the two surfaces of the sheet, which includes maintaining the two bodies of electrolyte substantially separated by the advancing sheet, while passing the flows of current from the sheet through the bodies respectively to separate electrodes on opposite sides of the sheet and while maintaining different electrical conditions for said flows of current whereby different predetermined densities of current are maintained on respectively opposite surfaces of the sheet to produce correspondingly differentanodic coatings.

8. A method of continuously differentially anodizing the opposite major surfaces of an elongated aluminum article having such surfaces, comprising advancing the article lengthwise in submerged contact with bodies of anodizing electrolyte or like composition at both surfaces of the article, While maintaining said bodies substantially separated by the article, and while passing current from the article through the electrolyte bodies respectively to separate electrodes on opposite sides of the article, for anodizing said surfaces, maintaining different electrical conditions for said flows of current to provide different predetermined densities of current on the opposite sur-' faces to produce correspondingly different anodic coat- 17 ings, and removing heat from said surfaces by turbulently advancing said bodies of electrolyte over the surfaces and continuously recirculating said bodies While removing heat therefrom.

9. A method as defined in claim 8, wherein the said electrodes are spaced by equal distances from the respective surfaces of the article and wherein said different electrical conditions are maintained by applying different voltages between the article and said electrodes.

10. A method as defined in claim 9, wherein said flows of current are supplied from a single current source and wherein said different voltages are applied by directing one of the flows through a path of higher resistance than the other.

1 8 References Cited UNITED STATES PATENTS FOREIGN PATENTS 9/1948 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner. JOHN H. MACK, Examiner.

15 T. TUFARIELLO, Assistant Examiner. 

1. A METHOD OF CONTINUOUSLY TREATING THE OPPOSITE SURFACES OF AN ELONGATED ALUMINUM ARTICLE TO PRODUCE AN ANODIC COATING THERON, COMPRISING ADVANCING SAID ARTICLE LENGTHWISE THROUGH A REGION OF SUBMERGED CONTACT OF SAID SURFACES WITH CLEANING ELECTROLYTE AND CONTINUOUSLY THEREAFTER THROUGH A REGION OF SUBMERGED CONTACT OF SAID SURFACES WITH ANODIZING ELECTROLYTE WHICH PRODUCES A POROUS COATING ON ALUMINUM, SAID ARTICLE BEING MAINTAINED IN SUBMERGED POSITION AND IN SUBSTANTIALLY CONTINUOUS CONTACT WITH ELECTROLYTE, BENEATH THE UPPER PARTS OF SAID REGIONS DURING ADVANCE BETWEEN THEM,WHILE PASSING ELECTRIC CURRENT THROUGH THE CLEANING ELECTROLYTE TO THE ARTICLE AS CATHODE IN THE FIRST REGION AND WHILE PASSING ELECTRIC CURRENT FRO THE ARTICLE, AS ANODE, INTO THE ANODIZING ELECTROLYTE IN THE SECOND REGION, SAID CURRENT IN THE SECOND REGON BEING PASSEED AT A DENSITY, OF AT LEAST 100 AMPERES PER SQUARE FOOT OF SAID SURFACES EXPOSED IN SAID SECOND REGION, FOR PRODUCING, IN COACTION WITH SAID ANODIZING ELECTROLYTE, A POROUS ANODIC COATING ON THE SURFACES, AND MAINTAING SUBSTANTIALLY CONSTANT TEMPERATURE CONDITIONS IN THE ELECTROLYTE BODIES ADJACENT THE ARTICLE SURFACES IN THE SAID REGIONS AS THE ARTICLE ADVANCES, BY TURBULENTLY ADVANCING THE ELECTROLYTE IN EACH REGION OVER THE SURFACES IN A DIRECTION LENGTHWISE OF THE PATH OF THE ARTICLE, AND RECIRCULATING THE ELECTROLYTE FROM EACH REGION THROUGH HEAT EXCHANGE PATHS FOR TEMPERATURE CONTROL THEREOF. 